1. Field of the Invention
The present invention relates to sensors. In particular, the present invention concerns micromechanically fabricated sensors that are manufactured on semiconductor substrates. Such sensors can be used for analyzing, for example, small amounts of biological matter. The invention also concerns a method for analyzing liquid phase samples micromechanically.
2. Description of Related Art
Using modern silicon micromachining techniques, it is possible to batch-fabricate micromechanical devices suitable for sensor applications. Many of these devices take advantage of silicon-on-insulator (SOI) structures. The recent research related to molecule-specific membranes [Vikholm1-4] has expanded the possibilities of micromechanically implemented biochemical analysis. It is well known that the biochemical analysis can be made by using micromechanical resonators, whose resonant frequency is altered by changes on the surface mass of the sensor. By monitoring the resonant frequency, information on the substance on the sensor is obtained.
In general, the resolution of detecting a small change in mass is inversely proportional to the effective mass of the resonator and directly proportional to the resonant frequency and the Q-value of the resonator. In addition, the resolution is proportional to the displacement amplitude of the resonator. Thus, the higher the mechanical energy stored by the resonator the better the resolution.
Surface wave sensors are based on exciting surface acoustic waves (SAWs) to the substrate. In such sensors, the wave propagates on (or in the vicinity of) the surface of the sensor, making them sensitive to small changes of mass on the surface of the sensor, for example, mass growth in gas environment. The SAW devices usually oscillate either in vertical or horizontal direction transverse to the propagation direction of the waves. The displacement of the surface of vertical mode SAW sensors is big, which makes them less suitable for analyzing of liquid samples. In particular, detection of small mass changes is difficult from liquid samples. This is because the oscillations are easily damped due to the vibrational energy radiated/lost to the liquid. Shear-Horizontal SAW (SH-SAW) sensors have been utilized as liquid phase sensors, though, as they utilize parallel-to-surface transverse waves. A SAW silicon micromechanical resonator utilizing a flexural wave mode can also be used but it is also suitable only for gas phase analysis.
Bulk Acoustic Waves (BAWs) propagate in the whole volume of the resonator. Thickness Shear Mode BAW Resonators (TSMs), which utilize transverse waves between the sensor electrodes, are well suited for gas analysis. They can also be used also for liquid phase analysis to some extent. Shear-Horizontal Acoustic Plate Mode Sensors (SH-APM) are more mass sensitive than TSMs but still much less sensitive than SAW devices. One principal reason for that is that the energy density on the surface of BAW sensors is lower than in SAW devices.
Almost any mechanical resonator can successfully be applied for measuring mass growth on the surface if placed in gas. When analyzing liquid samples, two problems arise. First, only sealed structures can be used. That is, the liquid-receiving section of the sensor has to be mechanically isolated from the other parts of the structure, such as transducer elements. Second, mechanical oscillations will be heavily damped by dissipations related to the liquid sample. Such dissipations result, for example, from relatively high masses of liquid-phase samples and viscous properties of the liquid, as well as from the relatively similar acoustic impedances of resonator materials and the liquid (typically within a decade). Thus, a significant amount of acoustic energy can be transmitted (lost) through the solid-liquid interface. This is why almost only sensors based on TSM, SH-APM, and SH-SAW, which all generate waves that propagate primarily in the shear horizontal (transverse, parallel-to-surface) direction, have been used as biosensors. However, the sensitivity and usability of the devices is not sufficient to meet the requirements of modern liquid phase analysis applications. A further disadvantage of prior art BAW devices is that the thickness of the resonator is dictated by the desired operational frequency.